1. Field of the Invention
The present invention relates to a racket frame and in particular, a tennis racket frame lightweight and excellent in its restitution performance.
2. Description of the Related Art
In recent years, there is proposed a so-called xe2x80x9cthick racketxe2x80x9d which is thick in an out-of-plane direction (ball-hitting direction) of the ball-hitting face of the racket frame. Females and seniors need the thick racket because they want to hit a tennis ball thereby at a high speed with a small force. The tennis racket they want is lightweight and excellent in ball-flying performance.
However, in consideration of a collision between the racket frame and a ball, the lightweight racket frame has a low restitution coefficient, according to the principle of the conservation of energy. That is, the light weight of the racket frame causes deterioration of its restitution performance. As a method of improving the restitution performance of the racket frame without increasing its weight, partly changing the rigidity of the racket frame [partly] is proposed.
For example, to improve the restitution performance of the racket frame, the tennis racket disclosed in Japanese Patent Application Laid-Open No.9-285569 has a highly rigid material extended from its face-side part to an extension direction of its face part to improve the rigidity value of the face-side part and reduce the deformation of the face part when a tennis ball is hit.
The present applicant proposed a racket frame as disclosed in Japanese Patent Application Laid-Open No.10-295855. The sectional peripheral length of the racket frame is constant, and only the sectional configuration of a particular portion is altered to reduce the moment of inertia of the section and increase the rigidity in the out-of-plane direction of the ball-hitting face. This racket frame has improved restitution performance.
However, the tennis racket disclosed in Japanese Patent Application Laid-Open No.9-285569 has the problem that although the tennis racket is lightweight, it does not have sufficient restitution performance, and the balance is great owing to the extension of the highly rigid material in the face-side part. Thus the tennis racket has a low operability. Also, the insertion of the highly rigid material into a part of the face part causes concentration of a stress and hence the strength of the racket frame deteriorates.
The racket frame disclosed in Japanese Patent Application Laid-Open No.10-295855 is lightweight and has preferable restitution performance. However since the sectional peripheral length of the racket frame is constant, it is impossible to change its rigidity value greatly. Thus there is room for improvement in its restitution performance.
The present invention has been made in view of the above-described demands. Thus, it is an object of the present invention to provide a racket frame that is lightweight without deteriorating its strength and superior in its restitution performance.
In order to achieve this object, according to the present invention, there is provided a pipe-shaped racket frame having a racket-frame body made of a fiber reinforced resin and composed of a string-stretched part (head), a throat part, a shaft part, and a grip part sequentially arranged and a yoke connected to the racket-frame body. In this construction, supposing that the head is a clock face and that a top position of the head is 12 o""clock, a yoke rigidity value at a central position, which is a vertical direction to a ball-hitting face, of the yoke in a longitudinal direction, is in a range of 10% to 70% of a face rigidity value which is an average of a rigidity value of a vertical direction to the ball-hitting face at a 12 o""clock position of the head and at a three o""clock position of the head.
As a result of their energetic researches, the present inventors found out that a racket frame that is lightweight and has improved restitution performance without deteriorating its strength is obtained by setting the yoke rigidity value at the central position of the yoke vertical to the ball-hitting face smaller than the face rigidity value which is the average of the rigidity value at the 12 o""clock position of the head vertical to the ball-hitting face and the rigidity value at the three o""clock position of the head vertical to the ball-hitting face and by integrally molding a fiber reinforced resin having a hollow portion to form the racket-frame body.
By setting the yoke rigidity value vertical to the ball-hitting face smaller than the face rigidity value vertical thereto, the head does not deform easily owing to its high rigidity, whereas the yoke flexes easily with the ball-hitting face, owing to its low rigidity, when a ball collides with the string-stretched ball-hitting face. Thereby vertical strings (guts) can be deformed greatly. Thus it is possible for the racket frame whose body is made of a fiber reinforced resin to have improved restitution performance without deteriorating its strength.
The yoke rigidity value is set not less than 10% of the face rigidity value nor more than 70% thereof, favorably not less than 30% thereof nor more than 70% thereof, and more favorably not less than 35% thereof nor more than 50% thereof.
If the yoke rigidity value is set less than 10% of the face rigidity value, the yoke has a low strength. On the other hand, if the yoke rigidity value is set more than 70% of the face rigidity value, the yoke is not flexible sufficiently and thus the restitution performance of the racket frame cannot be improved.
The yoke rigidity value is measured at the central position of the yoke in its longitudinal direction, vertical (frame thickness direction) to the ball-hitting face. As will be described later, the rigidity value is measured by a three-point bending method at the central position between two racket frame-supporting points (the central position in the longitudinal direction of the yoke) 70 mm (2.75 in.) apart from each other. Supposing that the head is a clock face, the top position of the head is 12 o""clock. The face rigidity value is the average of the rigidity value at the 12 o""clock position vertical to the ball-hitting face and the rigidity value at the three o""clock position vertical to the ball-hitting face measured in a method similar to the above method.
The yoke rigidity value is not less than 60 kgf/cm (338 lbf/in) nor more than 500 kgf/cm (2820 lbf/in), favorably not less than 100 kgf/cm (564 lbf/in) nor more than 450 kgf/cm (2538 lbf/in), and more favorably not less than 200 kgf/cm (1128 lbf/in) nor more than 350 kgf/cm 1974 lbf/in).
If the yoke rigidity value is set less than 60 kgf/cm (338 lbf/in), the yoke has a low strength. On the other hand, if the yoke rigidity value is set more than 500 kgf/cm (2820 lbf/in), the yoke is not flexible sufficiently and thus the restitution coefficient of the racket frame cannot be improved.
It is preferable that the yoke consists of a fiber reinforced resin, a resin, a metal, a wood or a composite material thereof.
As a metal, it is favorable to use lightweight metal such as aluminum, titanium, magnesium, and the like or alloys containing one of these lightweight metals as the main component. To allow the racket frame to have a high vibration-damping effect, it is more favorable to use a fiber-reinforced thermoplastic resin. As a matrix resin, polyamide resin and a mixture of polyamide and ABS resin are preferably used. As a reinforcing fiber, a short carbon fiber is preferably used.
The yoke is manufactured by a method of injection-molding a thermoplastic resin or the like reinforced with short carbon fibers or the like; a molding method of weaving co-mingled yarns of a polyamide fiber and a carbon fiber into a braid and fusing a polyamide to impregnate the reinforcing fiber therewith; or a method of forming RIM nylon by injecting a RIM nylon monomer into a laminate consisting of foamed epoxy, a nylon tube coating the foamed epoxy, and a carbon braid layered on the nylon tube.
To allow the racket-frame body to be lightweight and have a preferable rigidity and strength, it is preferable that the reinforcing fiber consists of a continuous fiber. The strength and rigidity of the racket-frame body maybe increased by composing the matrix resin of a thermosetting resin. The vibration-damping performance of the racket-frame body may be enhanced by composing the matrix resin of a thermoplastic resin. In view of the strength and rigidity of the racket-frame body, it is preferable that a carbon fiber is used as the reinforcing fiber and that an epoxy resin is used as the matrix resin. The fiber reinforced resin of the racket-frame body is arbitrarily selected according to the main function of the racket frame.
It is preferable that a groove is formed on the yoke at a ball-hitting face side thereof along a circumferential direction of the ball-hitting face. As the configuration of the groove, it is preferable that its width is in the range of 4 mm (0.16 in) to 6 mm (0.23), its depth is in the range of 4 mm (0.16 in) to 6 mm (0.23 in), and its length in the circumferential direction of the ball-hitting face is in the range of 20 mm (0.78 in) to 120 mm (4.7 in). Thereby it is possible to reduce the yoke rigidity value in the direction of the ball-hitting face and increase the movable range of the string by the depth of the groove, supposing that a tennis ball is hit in the same area of a racket frame having a groove and a racket frame lacking a groove. Thus the racket frame of the present invention having a groove has improved restitution performance.
It is preferable that the thickness of the yoke is not less than 10 mm (0.39 in) nor more than 25 mm (0.98 in) and its width is not less than 10 mm (0.39 in) nor more than 20 mm (0.78 in). if the thickness and width of the yoke are less than 10 mm (0.39 in), its strength is low. On the other hand, if the thickness and width of the yoke are more than 25 mm (0.98 in) and 20 mm respectively (0.75 in), its rigidity is high and thus it is difficult to improve the restitution performance of the racket frame. The length of the yoke (the horizontal distance between right and left points of connection between the yoke and the racket-frame body) is not less than 75 mm (2.9 in) nor more than 150 mm (5.8 in) and favorably not less than 85 mm (3.3 in) nor more than 120 mm (4.7 in). If the length of the yoke is less than the lower limit, the effect of its rigidity is low. On the other hand, if the length of the yoke is more than the upper limit, a tennis racket is too large. Thus a tennis racket has a problem in at least one of strength, weight, and operability.
The thickness of the head of the racket-frame body is not less than 18 mm (0.70) nor more than 30 mm (1.2 in) and favorably not less than 10 mm (0.39 in) nor more than 20 mm (0.78 in). If the thickness of the head is less than the lower limit, the rigidity of the head is insufficient. On the other hand, if the thickness of the head is more than the upper limit, the strength of the head is low. To allow the head to have a high strength in this case, it is necessary to increase the weight thereof, which leads to unfavorable operability of a tennis racket.
It is preferable that the racket-frame body and the yoke are formed separately and that the yoke and the racket-frame body are connected to each other by a mechanical connection means and/or an adhesive agent.
The material for the yoke and the material for the racket-frame body are not integrally molded but separately molded, and the molded material for the yoke and the molded material for the racket-frame body are connected to each other by a mechanical connection means. Therefore it is possible to secure the force of connecting the yoke and the racket-frame body to each other. Since the connection surface of the yoke and that of the racket-frame body are not integrated with each other, a shear load generated when the racket frame deforms is collectively applied to the connection surface of the yoke and that of the racket-frame body. Thereby vibrations generated on the entire racket frame are suppressed, which prevents occurrence of tennis elbow.
The connection portion of the yoke and that of the racket-frame body deform greatly in the primary and secondary vibrations in the out-of-plane direction. Thus the shear load can be easily collectively applied to the boundary between the yoke and the racket-frame body. Consequently it is possible to effectively suppress vibrations generated on the entire racket frame. Thus the racket frame of the present invention has a high vibration-damping performance.
A mechanical connection means connects objects to each other without the intermediary of a viscous material or a chemical connection force. A mechanical connection means is used to connect the objects to each other depending upon a difference in the configuration of the objects and a combination of variations thereof. Mechanical connection means include fit-on of a concavity and a convexity, screw-tightening, fitting, engagement, locking, bolt/nut, spring, and the like. Of these means, fit-on of a concavity and a convexity and screw-tightening are favorably used.
The mechanical connection means is required to hold a string force and withstand an impact force applied to the racket frame by a tennis ball. More specifically, a convexity is formed on the inner side of the racket-frame body or the connection surface of the yoke, while a concavity which fits on the convexity is formed on the inner side of the racket-frame body or the connection surface of the yoke. The yoke and the racket-frame body fit on each other by fit-on of the convexity and the concavity. In the case where the convexity is formed on the racket-frame body and the concavity is formed on the yoke, the restraint on the yoke relative to the racket-frame body is small. Thus it is easy to fit the yoke and the racket-frame body on each other. It is preferable that the racket-frame body has a depression corresponding to the configuration of the connection auxiliary part of the yoke to fittingly lock the connection auxiliary part and the racket-frame body to each other. Thereby it is possible to prevent both from shifting from each other and enhance the connection therebetween.
The right and left ends of the yoke are connected to the right and left parts of the racket-frame body respectively in not less than 10 cm2 (1.6 in2), favorably not less than 20 cm2 (3.2 in2), and more favorably not less than 30 cm2 (4.8 in2). If the area of the connection portion of the yoke and that of the racket-frame body is less than 10 cm2 (1.6 in2), a sufficient vibration-damping effect cannot be obtained. To increase the vibration-damping performance, it is desirable that the area of the connection portion is large. But in view of the strength and weight of the racket frame, the area of the connection portion is favorably less than 60 cm2 (9.6 in2). By changing the area of the connection portion, the vibration-damping performance can be controlled. Thus it is possible to appropriately set the vibration-damping factor according to player""s preference for a feeling (degree of vibration) the player has in hitting a tennis ball with a tennis racket.
An adhesive agent superior in vibration-absorbing property and/or a vibration-damping film or a vibration-damping sheet may be interposed on the boundary between the racket-frame body and the yoke. Furthermore a high vibration-damping material (vibration-damping film, vibration-damping sheet or vibration-damping paint) may be disposed on at least one portion of the boundary between the racket-frame body and the yoke. By selecting an appropriate vibration-damping material, it is easy to adjust the vibration-damping performance of the racket frame. The vibration-damping material maybe used singly or in combination with an adhesive agent.
The high vibration-damping material is particularly effective when the yoke and the racket-frame body are separately formed. In the case where an adhesive agent having a lower elastic modulus than the yoke and the racket-frame body is used in combination with a vibration-damping material, the effect of the adhesive force of the adhesive agent obtained in connecting both to each other is superior, and a shear stress is collectively applied to the boundary between the racket-frame body and the yoke. Therefore the racket frame has superior vibration-damping performance.
By interposing an adhesive agent and a vibration-damping material on the boundary between the racket-frame body and the yoke, it is possible to prevent generation of an unpleasant sound.
As the vibration-damping film, dipole gee film manufactured by C.C.I., Inc. is preferably used.
As the adhesive agent, those that are flexible are preferable. In addition to those composed of epoxy resin, those composed of urethane are preferable. Concrete examples are shown below.
A high separation-resistant and shock-resistant adhesive agent containing cyanoacrylate and elastomer as its base. For example, 1731xe2x80xa21733 produced by Three-Bond, Inc. is commercially available.
A cold-cure type two-pack epoxy resin having stable toughness formed by uniformly dispersing fine rubber particles in the epoxy resin. As an adhesive agent under a high shear, 2082C produced by Three-Bond, Inc. is commercially available.
An elastic adhesive agent of one-can moisture-cure type which contains a silyl group-containing specific polymer as its main component and hardens in reaction with a slight amount of water contained in air. For example, 1530 produced by Three-Bond, Inc. is commercially available.
A urethane resin adhesive agent: xe2x80x9cEsprenexe2x80x9d is commercially available.
xe2x80x9cRedux 609xe2x80x9d, xe2x80x9cAW106/HV953Uxe2x80x9d, and xe2x80x9cAW136A/Bxe2x80x9d produced by Ciba-Geigy, Inc. are commercially available.
xe2x80x9cE-214xe2x80x9d produced by LOCTITE, Inc. is commercially available.
xe2x80x9cDP-460xe2x80x9d and xe2x80x9c9323B/Axe2x80x9d produced by 3M, Inc. are commercially available.
It is preferable that the yoke has right and left connection auxiliary parts each extending from one end of the body of the yoke that closes an opening of the head, with each of the right and left connection auxiliary parts extending across the boundary between the head and the throat part, that each of the right and left connection auxiliary parts is extended up to a position of four o""clock (eight o""clock) of the head, supposing that the head is a clock face, and that the top position of the head is 12 o""clock, and each of the right and left connection auxiliary parts is extended up to the shaft part at the throat-part side.
The connection auxiliary part allows the yoke and the racket-frame body to be connected to each other in a large area and thus the connection surface of each of the yoke and the racket-frame body to easily receive a shear load. By collectively applying a stress to each of the connection surfaces, a high vibration-damping function can be easily displayed, and the yoke can be connected to the racket-frame body with a strong force.
The connection auxiliary part is extended up to the position of four o""clock (eight o""clock). The position of four o""clock (eight o""clock) is included in the loop of the secondary vibration mode. Thus the vibration-damping effect can be increased by extending the connection auxiliary part to the position of four o""clock (eight o""clock). When the connection auxiliary part is extended toward the position of 12 o""clock beyond the position of four o""clock, the racket frame has a large balance and the tennis racket has a low operability.
At the throat-part side, the connection auxiliary part may be extended to the shaft-part.
By adjusting the extension length of the connection auxiliary part to the head and to the throat part, the vibration-damping performance can be controlled and the balance point can be adjusted. Further by adjusting the extension length of the connection auxiliary part to the head, the area of the ball-hitting face can be also altered. Furthermore by altering the position of the body of the yoke to the top side of the entire racket frame or the grip side thereof, the area of the ball-hitting face of the racket frame can be easily altered.
Each of the right and left connection auxiliary parts has an equal and uniform dimension in one region and a nonuniform dimension in other region in a thickness direction thereof. The dimension of the connection auxiliary part in its thickness direction is set smaller than that of the racket-frame body in its thickness direction to prevent the connection auxiliary part from projecting from the racket-frame body.
By making the dimension of the connection auxiliary part in its thickness direction nonuniform, it is possible to fit the convexity of the racket-frame body and the concavity of the connection auxiliary part (or the concavity of the racket-frame body and the convexity of the connection auxiliary part) on each other with a higher force and make the connection auxiliary part look attractive.
Preferably, each of the right and left connection auxiliary parts of the yoke is extended to the shaft part along an inner surface of the throat part in such a way that a leading end of the right connection auxiliary part is continuous with that of the left connection auxiliary part to form an approximately hollow triangular space with the connection auxiliary part and the body of the yoke. This configuration increases the strength of the yoke.
It is preferable that the yoke has a projection projected from a portion at which the leading end of the right connection auxiliary part is continuous with the leading end of the left connection auxiliary part toward the shaft part. It is preferable that the projection is inserted into a slit formed at a center of a leading end of the shaft part. By inserting the projection into the slit formed on the shaft part, it is easy to dispose the yoke at a predetermined position of the racket-frame body and connect the yoke and the racket-frame body to each other in a large area to thereby enhance the vibration-damping performance of the racket frame.
Both ends of the body of the yoke and a connection auxiliary part extending from the both ends of the body of the yoke are connected to an inner-surface side of the racket-frame body by superimposing an outer surface of the connection auxiliary part and an inner surface of the racket-frame body on each other (former construction). Otherwise, the yoke and the racket-frame body are connected to each other by fitting the connection auxiliary part on a fit-on portion formed on the inner surface of the racket-frame body in correspondence to a configuration of the connection auxiliary part (latter construction).
The former construction is larger in the area of the contact between the yoke and the racket-frame body than the latter construction. The latter construction allows the racket frame to be lightweight.
The weight of the yoke is set to the range of favorably 5%-30% and more favorably 10%-25% of the weight of a raw frame whose weight is the addition of the weight of the yoke and that of the racket-frame body.
If the weight of the yoke is less than 5% of the weight of the raw frame, the yoke has a low strength. On the other hand, if the weight of the yoke is more than 30% of the weight of the raw frame, the weight of the yoke is too large.
The weight of the racket frame is not less than 100 g (0.22 lbs) nor more than 280 g (0.62 lbs) and favorably not less than 200 g (0.44 lbs) nor more than 260 g (0.57 lbs). If the weight of the racket frame is less than 100 g (0.22 lbs), a tennis racket has an insufficient strength. On the other hand, if the weight of the racket frame is more than 280 g (0.62 lbs), the weight of the tennis racket cannot be reduced. The weight of the racket frame means the weight of a finished product (the weight of paint and that of the grip part) of the racket frame not having strings mounted thereon.
The resin for use in the racket frame of the present invention includes a thermosetting resin and a thermoplastic resin, as described above. A thermosetting resin includes epoxy resin, unsaturated polyester resin, phenol resin, melamine resin, urea resin, diallyl phthalate resin, polyurethane resin, polyimide resin, and silicon resin.
A thermoplastic resin includes polyamide resin, saturated polyester resin, polycarbonate resin, ABS resin, polyvinyl chloride resin, polyacetal resin, polystyrene resin, polyethylene resin, polyvinyl acetate, AS resin, methacrylate resin, polypropylene resin, and fluorine resin.
As reinforcing fibers for use in the fiber reinforced resin, fibers which are used as high-performance reinforcing fibers can be used. For example, it is possible to use carbon fiber, graphite fiber, aramid fiber, silicon carbide fiber, alumina fiber, boron fiber, glass fiber, aromatic polyamide fiber, aromatic polyester fiber, ultra-high-molecular-weight polyethylene fiber, and the like. Metal fibers may be used as the reinforcing fiber. A carbon fiber is preferable because it is lightweight and has a high strength. These reinforcing fibers can be used in the form of long or short fibers. A mixture of two or more of these reinforcing fibers may be used. The configuration and arrangement of the reinforcing fibers are not limited to specific ones. For example, they may be arranged in a single direction or a random direction. The reinforcing fibers may have the shape of a sheet, a mat, fabrics (cloth), braids, and the like.
The racket-frame body is not limited to a laminate of fiber reinforced prepregs. The racket-frame body may be formed by winding reinforcing fibers on a mandrel by filament winding to form a layup, disposing the layup in a die, and filling the thermoplastic resin such as rim nylon into the die.